Fwcutter allows you to extract the firmware required for Broadcom 43xx chips
out of the .o files available with the card or on the internet.
Not all versions of the firmwares are supported by this tool.
You can list supported firmwares with :
b43-fwcutter -l
Depending on your chip, you will need a version 3 or 4 firmware.
Useful reading and links to firmwares that can be extracted are available at
http://linuxwireless.org/en/users/Drivers/b43
Extract (as root) with :
b43-fwcutter -w /lib/firmware wl_apsta_mimo.o
It will create files directly at the required place (in either b43legacy or
b43 subfolder).